How does dehydration affect aldosterone secretion

Water-electrolyte balance

Synonym: water and electrolyte balance

1 definition

As Water-electrolyte balance is the name given to the physiological system of uptake and release of water and the closely related regulation of the concentration of electrolytes, i.e. positively and negatively charged, dissolved particles. The water-electrolyte balance determines the fluid distribution in the human body and is an indispensable basis for all life processes.

2 basics

2.1 Distribution of body water

The water content in an adult man is 60% of the body weight, in a woman 50% and in an infant 75%. Two thirds of the total body water is intracellular, and two thirds of the remaining third are interstitial.

A distinction is made between the following compartments, which are separated by cell or basement membranes:

The water content differs significantly between the individual tissues, e.g .:

2.2 electrolytes

Electrolytes are small charged particles (ions or dissociated salts). The most important electrolytes in the body are

  • the positively charged cations:
    • N / A+ (Sodium): especially extracellular
    • K+ (Potassium): mainly intracellular
    • Approx2+ (Calcium)
    • Mg2+ (Magnesium)
  • the negatively charged anions:
    • Cl- (Chloride)
    • HCO3- (Bicarbonate)
    • PO43− (Phosphate)
    • other negatively charged particles, including larger ones, e.g. proteins

Electrolytes are available intracellular, interstitial and intravascular in different concentrations:

  • intracellular: especially K+- and phosphate ions
  • extracellular: especially Na+-, Cl-- as well as bicarbonate ions. These electrolyte ratios are maintained in the cell membranes by active ion pumps.

Since the electrolytes are particles that conduct electrical voltage, the voltage on the cell membranes also changes depending on the electrolyte concentration. The electrical voltage on the cell membrane controls a large number of processes that take place at the cell level. In this way, electrolytes not only determine the distribution of fluids in the body, but also cellular functions, e.g. the depolarization of nerve cells when a nerve stimulus is passed on.

2.3 Intra- and extracellular volumes

The distribution of total body water in different compartments across the membranes is determined by the osmotic pressure. Because water can diffuse freely through the membranes, the osmotic pressure of the intracellular and extracellular spaces is balanced despite the different composition of the compartments. If there is a different osmolarity in one compartment than in another, water diffuses through the cell membrane and balances out the osmotic pressure difference. In doing so, however, the volume of water in this compartment increases.

Since there are high protein concentrations in the cell and these macromolecules cannot pass through the cell membrane, water would flow into the interior of the cell from the extracellular space due to the colloid osmotic pressure. The consequence would be cell swelling and cell death (Gibbs-Donnan equilibrium). Therefore, the cell has a sodium-potassium-ATPase, which uses energy to transport 3 sodium ions into the extracellular space and 2 potassium ions into the intracellular space. At the same time, the resulting chemical gradient of the potassium ion creates a membrane potential.

The blood plasma (intravascular fluid) is also rich in protein compared to the interstitial fluid. Here, too, the colloid osmotic forces would lead to the influx of water into the intravascular space, but the intravascular hydrostatic pressure counteracts this water movement.

3 physiology

The absorption of water and electrolytes occurs orally via the digestive tract, which absorbs fluids and ions through the intestinal wall. Excretion occurs through the kidneys (diuresis), through the skin in the form of perspiration (sweating) and through the air we breathe. The relationship between the various excretion routes depends, among other things, on climatic conditions. The relationship between water absorption and excretion is recorded in the fluid balance.

The amount of electrolyte excretion or retention in the kidney must always be adjusted to the needs: Through urine, stool, perspiration, etc., water and electrolytes are lost every day. Water and electrolytes are added through drinking, eating, possibly infusions and oxidizing water (in the case of fever, hyperthyroidism or after surgery).

The body regulates the water-electrolyte balance primarily through two mechanisms:

  • Determination of the osmolality
  • Detection of volume deviations

3.1 Osmolality

The slightest changes in plasma osmolality are registered by osmoreceptors in the hypothalamus, which are located in the circumventricular organs. These neurons contain stretch-inactivating cation channels. When the extracellular osmolality increases, these cells shrink, so that depolarization occurs. As a result, the antidiuretic hormone (ADH) is released in the neurons of the supraoptic and paraventricular nucleus.

see also: Osmoregulation

3.2 Volume measurement

Changes in volume are determined primarily via stretch receptors, especially in the vein openings to the right and left atrium as well as in the area of ​​the portal vein. The result of the excitation of volume-sensitive receptors is the inhibition of ADH release and renal sympathetic innervation. The decreased ADH release in the case of atrial distension is mediated by afferents of the vagus nerve. This Gauer-Henry reflex leads to increased renal fluid excretion within minutes.

When the wall tension in the auricles increases, the atrial natriuretic peptide (ANP) is also released there. It leads to natriuresis within seconds to minutes by stimulating kidney perfusion and inhibiting sodium absorption in the collecting tube. In addition, ANP directly and indirectly inhibits renin secretion. This results in decreased aldosterone secretion from the adrenal glands. ANP also directly inhibits aldosterone synthesis and release. A similar hormone is also produced in the brain (BNP), where it lowers blood pressure, inhibits the release of ADH and the sympathetic system, and decreases the feeling of thirst. The primary purpose of these mechanisms is to counteract volume and pressure overloading of the heart and the circulatory system.

Volume regulation also takes place via pressoreceptors in the carotid sinus and aortic arch. Only significant changes in volume (e.g. in the event of blood loss) affect the high pressure system. In these situations, the plasma concentration of ADH is extremely elevated. In this case, ADH also leads to vasoconstriction via V1 receptors.

3.3 Sodium balance

The body's sodium content is primarily regulated by the kidneys (mainly by aldosterone). The average sodium content in adults is around 4,000 mmol. About 40% of this is in the bone. More than two thirds of this share is bound to crystalline structures and therefore not easily exchangeable. The rest, like the sodium of the plasma, the interstitium and the cell, is available for exchange.

Sodium is largely absorbed with food in the form of table salt. The daily amount absorbed varies between a few mmol up to 1,000 mmol, on average it is 100 mmol / d. Under normal circumstances, sodium is mainly excreted through the kidneys; only small amounts are eliminated through stool and sweat.

The body measures the total amount of body sodium indirectly via changes in the volume of the extracellular space. Salt excretion is primarily determined by the renin-angiotensin-aldosterone system, with the sympathetic and natriuretic peptides having a modulating effect.

3.4 Volume excretion

While the RAAS regulates the maintenance of extracellular volume through renal sodium resorption, osmolality and water uptake and excretion are primarily controlled by thirst and ADH.

3.5 thirst

The feeling of thirst arises from:

  • Increase in plasma osmolality (osmotic thirst)
  • Decrease in extracellular fluid volume (hypovolemic thirst)

While osmotic thirst usually dominates, loss of volume and salt lead to water and salt uptake via renal hormonal and central mechanisms. RAAS, baroreceptors and volume receptors, ADH, oxytocin and hypothalamic centers are involved.

4 clinic

In certain situations (e.g. with severe diarrhea, vomiting, sweating, blood loss) or with various diseases (renal insufficiency), the body's own regulation by the kidneys may be insufficient or fail. This then leads to disturbances in the water-electrolyte balance, which can be objectified by determining the serum electrolytes in a laboratory.

These include:

These conditions can sometimes be life-threatening and must - depending on the severity - be treated immediately medically, e.g. by hydration or infusions of solutions containing electrolytes.

Isotonic changes in fluid levels are limited to the extracellular space. However, if the osmolality increases (hypertonic deflection), water is moved out of the cells, resulting in cell shrinkage. Conversely, cells swell when there is a hypotonic change.

4.1 Dehydration

Dehydration is rare as long as you drink enough water. The feeling of thirst usually provides a balance, even with insufficient ADH activity (diabetes insipidus). Dehydration is mainly found in older people who have a reduced sense of thirst, as well as in people who have limited mobility. Triggering events are often diarrhea, vomiting, burns or the use of diuretics.

There are three types of dehydration:

  • Hypotonic dehydration: Sodium loss is greater than water loss (e.g. diarrhea, vomiting) or when compensating for volume loss through drinking hypoosmolar fluids and with restricted ability to retain salt (e.g. in hypoaldosteronism)
  • isotonic dehydration: loss of sodium and water in equal proportions (e.g. with bleeding or peritonitis)
  • Hypertonic dehydration: water loss is greater than sodium loss or if the fluid loss is not compensated for. Especially if you work hard in the heat or if you have a fever, as a lot of hypo-osmolar fluid is lost.

4.2 hyperhydration

There are also three types of hyperhydration:

A hyperhydration usually requires a disturbance of the water excretion (kidney failure or hyperaldosteronism)

4.3 Hyponatremia

In hyponatremia, a distinction is made between:

4.4 Hypernatremia

Analogous to hyponatremia, a distinction is also made between three forms: